1. Field of the Invention
The invention relates pressure boosters and particularly to a pressure booster for a fuel injection system for internal combustion engines.
2. Description of the Prior Art
As exhaust gas standards become increasingly stringent, ever-higher injection pressures are required to improve mixture formation and combustion. The result is greater mechanical and thermal stresses on the fuel injection system. Furthermore, the demand for driving power is increasing disproportionately, since with the pressure, the losses in the fuel injection system rise as well.
The object of the invention is to furnish a fuel injection system in which the hydraulic forces acting on the pressure booster are reduced, and in which the pressure booster is simple to control. In addition, higher injection pressures should be made possible and at the same time the stress and the demand for driving power on the injection pump should be reduced.
This object is attained according to the invention by a pressure booster for a fuel injection system for internal combustion engines, having an injection nozzle and having an injection pump that has a high-pressure part, the high-pressure part of the injection pump being in operative communication with the injection nozzle via a control line communicating with the low-pressure side of the pressure booster and via a high-pressure path communicating with the high-pressure side of the pressure booster, characterized in that means to compensate for the hydraulic force acting on the low-pressure side of the pressure booster between injections are present.
This pressure booster has the advantage that between injections, the pressure forces acting on the stepped piston are reduced, so that after the end of the injection, the stepped piston can be returned with less force to its outset position toward the pump, and the pressure booster can be controlled in a simple way. The leakage and throttling losses of the fuel injection system are reduced as well, which leads to a reduction in the demand for driving power and improves the hydraulic efficiency of the fuel injection system. In addition, the high injection pressures allow smaller injection port diameters for the injection nozzle, which improves the mixture formation at all operating points.
In a feature of the pressure booster of the invention, it is provided that the pressure booster has a stepped piston, which is displaceable in a bore and whose end faces each define one pressure chamber; that a first, larger end face of the stepped piston defines a first pressure chamber, communicating with the control line; that a second, opposed, smaller end face of the stepped piston defines a second pressure chamber, communicating with the high-pressure path; that an annular face opposite the first end face defines a control chamber; and that between injections, the pressure is the same in both the first pressure chamber and the control chamber. This version has the advantage that despite a simple design, it has the advantages of the invention.
In a further feature of the invention, it is provided that the first end face and the annular face are essentially the same size, so that the hydraulic forces on the stepped piston are compensated for entirely between injections.
Another embodiment provides that the control chamber communicates hydraulically with the first pressure chamber via an inlet throttle, so that during the injection, pressure equalization between the first pressure chamber and the control chamber is prevented.
In one feature of the invention, the control chamber can be pressure-relieved via a control valve, in particular a 2/21-way control valve, so that the onset of supply by the pressure booster is controllable by opening the control valve.
A further variant provides that a restoring spring is fastened in the first pressure chamber and is braced on a support and a shoulder of the stepped piston and as a function of the pressure in the first pressure chamber and the pressure in the control chamber presses the stepped piston, between injections, against its stop toward the pump, so that after the end of injection, the stepped piston is put into its outset position regardless of the pressures that prevail. Furthermore, the restoring spring requires only little installation space.
In a feature of the invention, the shoulder is separably connected to the stepped piston, thus simplifying production and assembly.
In a further feature of the invention, the fuel flows out of the first pressure chamber into a leaking oil return via an outlet throttle, so that the pressure level in the leaking oil return is created indirectly by the injection pump.
A further variant of the invention provides that the second pressure chamber is filled from the leaking oil return, and that a check valve, which blocks the return flow of fuel from the second pressure chamber to the leaking oil return, is disposed between the second pressure chamber and the leaking oil return, so that the filling is simple, and only a low pressure prevails in the second pressure chamber between injections.
Further in the invention, it is provided that the pressure in the leaking oil return is less than the opening pressure of the injection nozzle, so that the injection nozzle closes securely between injections.
In one feature of the invention, to simplify production and assembly, the stepped piston is embodied in two parts.
One version of the invention provides that the control quantity of the control valve is carried away into the leaking oil return, so that a simple hydraulic circuit is attained.
In another variant, it is provided that the control piston is guided in a sleeve, so that the guidance of the stepped piston is improved.